Number Of Asteroids Research Articles

Active control of the visual field in the starfish Acanthaster planci

Photoreception in echinoderms has been studied for several years with a focus on the dermal photoreceptors of echinoids. Even though spatial vision has been proposed for this dermal photosystem, by far the most advanced system is found in a number of asteroids where an unpaired tube foot at the tip of each arm carries a proper eye, also known as the optical cushion. The eyes resemble compound eyes, except for the lack of true optics, and they typically have between 50 and 250 ommatidia each. These eyes have been known for two centuries but no visually guided behaviors were known in starfish until recently when it was shown that both Linckia laevigata and Acanthaster planci navigate their coral reef habitat using vision. Here we investigate the visual system of A. planci and find that they have active control of their visual field. The distalmost tube foot holding the eye is situated on a movable knob, which bends to adjust the vertical angle of the visual field. On the leading arms the visual field is directed 33° above the horizon, whereas the eyes on the trailing arms are directed 44° above horizontal on average. When the animal traverses an obstacle the knob bends and counteracts most of the arm bending. Further, we examined a previously described behavior, rhythmic arm elevation, and suggest that it allows the animal to scan the surroundings while preventing photoreceptor adaptation and optimizing image contrast.

Predictions for Dusty Mass Loss from Asteroids During Close Encounters with Solar Probe Plus

The Solar Probe Plus (SPP) mission will explore the Sun's corona and innermost solar wind starting in 2018. The spacecraft will also come close to a number of Mercury-crossing asteroids with perihelia less than 0.3 AU. At small heliocentric distances, these objects may begin to lose mass, thus becoming "active asteroids" with comet-like comae or tails. This paper assembles a database of 97 known Mercury-crossing asteroids that may be encountered by SPP, and it presents estimates of their time-dependent visible-light fluxes and mass loss rates. Assuming a similar efficiency of sky background subtraction as was achieved by STEREO, we find that approximately 80% of these asteroids are bright enough to be observed by the Wide-field Imager for SPP (WISPR). A model of gas/dust mass loss from these asteroids is developed and calibrated against existing observations. This model is used to estimate the visible-light fluxes and spatial extents of spherical comae. Observable dust clouds occur only when the asteroids approach the Sun closer than 0.2 AU. The model predicts that during the primary SPP mission between 2018 and 2025, there should be 113 discrete events (for 24 unique asteroids) during which the modeled comae have angular sizes resolvable by WISPR. The largest of these correspond to asteroids 3200 Phaethon, 137924, 155140, and 289227, all with angular sizes of roughly 15 to 30 arcminutes. We note that the SPP trajectory may still change, but no matter the details there should still be multiple opportunities for fruitful asteroid observations.

Footprints of a possible Ceres asteroid paleo-family

Ceres is the largest and most massive body in the asteroid main belt. Observational data from the Dawn spacecraft reveal the presence of at least two impact craters about 280~km in diameter on the Ceres surface, that could have expelled a significant number of fragments. Yet, standard techniques for identifying dynamical asteroid families have not detected any Ceres family. In this work, we argue that linear secular resonances with Ceres deplete the population of objects near Ceres. Also, because of the high escape velocity from Ceres, family members are expected to be very dispersed, with a considerable fraction of km-sized fragments that should be able to reach the pristine region of the main belt, the area between the 5J:-2A and 7J:-3A mean-motion resonances, where the observed number of asteroids is low. Rather than looking for possible Ceres family members near Ceres, here we propose to search in the pristine region. We identified 156 asteroids whose taxonomy, colors, albedo could be compatible with being fragments from Ceres. Remarkably, most of these objects have inclinations near that of Ceres itself.

Efficient spin sense determination of Flora-region asteroids via the epoch method

The Flora asteroid family’s size and location on the inner edge of the main belt make it a likely source of NEOs and terrestrial planet impactors; however, reliable determination of Flora membership is inhibited by the family’s age and the presence of a high density of background objects. Dykhuis et al. (Dykhuis et al. [2014]. Icarus 243, 111–128) identified the Flora family as the product of a 950-My-old collision dispersed in semimajor axis as a result of the Yarkovsky effect, and defined the family’s membership and extent in orbital parameter space. The observed preponderance of prograde rotators at semimajor axes greater than that of (8) Flora is consistent with the predictions of the single-collision Yarkovsky dispersion model.Here we extend the available rotational property data for the Flora family via a survey of 21 Flora-region asteroids, using a time-efficient modification of the “epoch method” to determine prograde/retrograde spin sense. Five of the survey asteroids are shown to be prograde; five are shown to be retrograde; six are shown to have spin axes in or near their orbital planes; and five represent other cases for which spin axis information was not determined. The high-semimajor axis component of the Flora family is found to have only prograde and in-plane rotators, consistent with model predictions of Yarkovsky dispersion. Moreover, we confirm a wide range of ecliptic latitudes of the spin axes among these prograde rotators, consistent with models of family evolution in which a significant fraction of the members are captured in spin–orbit resonance. Near the “center” of the family (near the semimajor axis location of (8) Flora), the spin directions are mixed, with a slight preference for retrograde rotators, placing constraints on the efficiency of YORP-cycle spin reorientation for the family.In addition to our spin sense survey, we also report new measurements of the Sloan colors of a number of large inner main belt asteroids.

Total mass of the Jupiter Trojans

The total mass of the Jupiter Trojans is estimated at (0.30 ± 0.19) × 10–10MSun using all available physical characteristics of these asteroids. The mass of asteroids in the L4 swarm ((0.19 ± 0.11) × 10–10MSun) is higher than that in the L5 swarm ((0.11 ± 0.07) × 10–10MSun) by a factor of 1.7. The obtained estimates include the hidden mass of asteroids that are not discovered yet. This hidden component constitutes 7% of the total mass. The number of Trojans with diameters larger than 1 km is estimated at 6 × 105. The number of such asteroids at the L4 Lagrangian point (NL4 = 4 × 105) is two times higher than that at the L5 Lagrangian point (NL5 = 2 × 105). It is found that the NL4/NL5 ratio is increased at smaller asteroid sizes or higher absolute magnitudes H. This ratio equals just 1.3 for asteroids with H 30 km). The effect of gravitational perturbations from Trojans on the motion of Hildian asteroids and centaurs is evaluated. This effect turned out to be at the limit of accuracy of modern observations.

The Explored Asteroids: Science and Exploration in the Space Age

Interest in asteroids is currently high in view of their scientific importance, the impact hazard, and the in situ resource opportunities they offer. They are also a case study of the intimate relationship between science and exploration. A detailed review of the twelve asteroids that have been visited by eight robotic spacecraft is presented here. While the twelve explored asteroids have many features in common, like their heavily cratered and regolith covered surfaces, they are a remarkably diverse group. Some have low-eccentricity orbits in the main belt, while some are potentially hazardous objects. They range from dwarf planets to primary planetesimals to fragments of larger precursor objects to tiny shards. One has a moon. Their surface compositions range from basaltic to various chondrite-like compositions. Here their properties are reviewed and what was confirmed and what was newly learned is discussed, and additionally the explored asteroids are compared with comets and meteorites. Several topics are developed. These topics are the internal structure of asteroids, water distribution in the inner solar system and its role in shaping surfaces, and the meteoritic links. It is suggested, that asteroid-scale grooves, ridges, and catenas on several explored asteroids argue against these asteroids having rubble pile interiors, i.e. interiors made when impact fragments reaccumulate. The only body for which this is not true is the tiny Itokawa and it is argued that this asteroid is a regolith breccia. The discovery of water on Vesta, fluidization textures on comets and possibly Eros, and the relatively large number of active asteroids inside the purported snowline, suggests that significant subsurface water may be present on asteroids in the inner solar system and may partly account for their low densities. The explored asteroids have also confirmed the linkage of the HED meteorites with Vesta and Itokawa with the ordinary chondrite meteorites, Eros is somewhat problematical. So while diversity, and the range of sizes, histories, and surface compositions, is the hall mark of the explored asteroids, the number of explored asteroids is small compared with the diversity of material expected on the basis of asteroid astronomy and meteorite geochemistry. The exploration of the solar system’s asteroids has only just begun.

Direct Detections of the Yarkovsky Effect: Status and Outlook

AbstractWe report the current results on a comprehensive scan of the near-Earth asteroid catalog for evidence of the Yarkovsky effect in the orbital motion of these bodies. While most objects do not have sufficient observational data to reveal such slight acceleration, we do identify 42 asteroids with a “valid” detection of the Yarkovsky effect, i.e., those with a signal at least 3 times greater than the formal uncertainty and a value compatible with the Yarkovsky mechanism.We also identify a special category of non-detection, which we refer to as “weak signal,” where the objects are of a size that would permit a clear detection if the Yarkovsky effect is maximized, and yet the orbit is clearly incompatible with such accelerations. The implication is that the Yarkovsky effect is reduced in these cases, presumably due to mid-range obliquity, but possibly also due to size, bulk density, thermal inertia, albedo, or spin rate markedly different from assumptions.Finally, there are a number of asteroids showing a significant signal for nongravitational acceleration, and yet with a magnitude too great to be attributed to the Yarkovsky effect. We term these “spurious detections” because most are due to erroneous optical astrometry, often involving a single isolated night from precovery observations. Some cases may be due to other nongravitational accelerations, such as outgassing, mass loss, or micro-meteoroid flux.

Characterizing asteroids multiply-observed at infrared wavelengths

We report Markov chain Monte Carlo fits of the thermophysical model of Wright (Wright, E.L. [2007]. Astrophysics e-prints arXiv:astro-ph/0703058) to the fluxes of 10 asteroids which have been observed by both WISE and NEOWISE. This model is especially useful when one has observations of an asteroid at multiple epochs, as it takes advantage of the views of different local times and latitudes to determine the spin axis and the thermal parameter. Many of the asteroids NEOWISE observes will have already been imaged by WISE, so this proof of concept shows there is an opportunity to use a rotating cratered thermophysical model to determine surface thermal properties of a large number of asteroids.

MAIN-BELT ASTEROIDS IN THE K2 ENGINEERING FIELD OF VIEW

Unlike NASA's original Kepler Discovery Mission, the renewed K2 Mission will stare at the plane of the Ecliptic, observing each field for approximately 75 days. This will bring new opportunities and challenges, in particular the presence of a large number of main-belt asteroids that will contaminate the photometry. The large pixel size makes K2 data susceptible to the effect of apparent minor planet encounters. Here we investigate the effects of asteroid encounters on photometric precision using a sub-sample of the K2 Engineering data taken in February, 2014. We show examples of asteroid contamination to facilitate their recognition and distinguish these events from other error sources. We conclude that main-belt asteroids will have considerable effects on K2 photometry of a large number of photometric targets during the Mission, that will have to be taken into account. These results will be readily applicable for future space photometric missions applying large-format CCDs, such as TESS and PLATO.

Additional criteria for identifying the asteroid families and confirmation of the effect of spatial separation of family members according to their albedos

The D(a) distribution of asteroid sizes by their semimajor axes and the N(p) distribution of the number of asteroids by their albedo values for individual families are used to isolate the asteroid families more clearly. The families identified by Masiero et al. (2013) are analyzed with the use of these distributions, and correctly and incorrectly isolated families are found. A reduction in the mean albedo with increasing semimajor axis is observed for almost all correctly identified families that are not truncated by resonances. This reduction is statistically significant for the majority of these families. Not a single family exhibits a statistically significant increase in albedo. This confirms our previous conclusions that a nongravitational effect acting in the asteroid belt results in the spatial separation of asteroids with different albedos.

Excluding interlopers from asteroid families

AbstractTo study an asteroid family it is crucial to determine reliably the list of its members, i.e. to reduce the number of interlopers as much as possible. However, as the number of known asteroids increases fast it becomes more and more difficult to obtain robust list of members of an asteroid family. To cope with these challenges we are proposing a new approach that may help to significantly reduce presence of interlopers among the family members.

PHYSICAL PROPERTIES OF ASTEROIDS IN COMET-LIKE ORBITS IN INFRARED ASTEROID SURVEY CATALOGS

We investigated the population of asteroids in comet-like orbits using available asteroid size and albedo catalogs of data taken with the Infrared Astronomical Satellite, AKARI, and the Wide-field Infrared Survey Explorer on the basis of their orbital properties (i.e., the Tisserand parameter with respect to Jupiter, TJ, and the aphelion distance, Q). We found that (i) there are 123 asteroids in comet-like orbits by our criteria (i.e., Q < 4.5 AU and TJ < 3), (ii) 80% of them have low albedo, pv < 0.1, consistent with comet nuclei, (iii) low-albedo objects among them have a size distribution shallower than that of active comet nuclei, that is, the power index of the cumulative size distribution of around 1.1, (iv) unexpectedly, a considerable number (i.e., 25 by our criteria) of asteroids in comet-like orbits have high albedo, pv > 0.1. We noticed that such high-albedo objects mostly consist of small (D < 3 km) bodies distributed in near-Earth space (with perihelion distance of q < 1.3 AU). We suggest that such high-albedo, small objects were susceptible to the Yarkovsky effect and drifted into comet-like orbits via chaotic resonances with planets.

Asteroid families classification: Exploiting very large datasets

The number of asteroids with accurately determined orbits increases fast, and this increase is also accelerating. The catalogs of asteroid physical observations have also increased, although the number of objects is still smaller than in the orbital catalogs. Thus it becomes more and more challenging to perform, maintain and update a classification of asteroids into families. To cope with these challenges we developed a new approach to the asteroid family classification by combining the Hierarchical Clustering Method (HCM) with a method to add new members to existing families. This procedure makes use of the much larger amount of information contained in the proper elements catalogs, with respect to classifications using also physical observations for a smaller number of asteroids.Our work is based on a large catalog of high accuracy synthetic proper elements (available from AstDyS), containing data for >330,000 numbered asteroids. By selecting from the catalog a much smaller number of large asteroids, we first identify a number of core families; to these we attribute the next layer of smaller objects. Then, we remove all the family members from the catalog, and reapply the HCM to the rest. This gives both satellite families which extend the core families and new independent families, consisting mainly of small asteroids. These two cases are discriminated by another step of attribution of new members and by merging intersecting families. This leads to a classification with 128 families and currently 87,095 members. The number of members can be increased automatically with each update of the proper elements catalog; changes in the list of families are not automated.By using information from absolute magnitudes, we take advantage of the larger size range in some families to analyze their shape in the proper semimajor axis vs. inverse diameter plane. This leads to a new method to estimate the family age, or ages in cases where we identify internal structures. The analysis of the plot above evidences some open problems but also the possibility of obtaining further information of the geometrical properties of the impact process. The results from the previous steps are then analyzed, using also auxiliary information on physical properties including WISE albedos and SDSS color indexes. This allows to solve some difficult cases of families overlapping in the proper elements space but generated by different collisional events.The families formed by one or more cratering events are found to be more numerous than previously believed because the fragments are smaller. We analyze some examples of cratering families (Massalia, Vesta, Eunomia) which show internal structures, interpreted as multiple collisions. We also discuss why Ceres has no family.

A comparative study of infrared asteroid surveys: IRAS, AKARI, and WISE

Abstract We present a comparative study of three infrared asteroid surveys based on the size and albedo data from the Infrared Astronomical Satellite (IRAS), the Japanese infrared satellite AKARI, and the Wide-field Infrared Survey Explorer (WISE). Our study showed that: (i) the total number of asteroids detected with diameter and albedo information with these three surveyors is 138285, which is largely contributed by WISE; (ii) the diameters and albedos measured by the three surveyors for 1993 commonly detected asteroids are in good agreement, and within ±10% in diameter and ±22% in albedo at 1 σ deviation level. It is true that WISE offers size and albedo of a large fraction (&amp;gt; 20%) of known asteroids down to bodies of a few km, but we would suggest that the IRAS and AKARI catalogs compensate for larger asteroids up to several hundred km, especially in the main belt region. We discuss the complementarity of these three catalogs in order to facilitate the use of these data sets for characterizing the physical properties of minor planets.

Vertex Ordering Characterizations of Graphs of Bounded Asteroidal Number

Asteroidal Triple-free (AT-free) graphs have received considerable attention due to their inclusion of various important graphs families, such as interval and cocomparability graphs. The asteroidal number of a graph is the size of a largest subset of vertices such that the removal of the closed neighborhood of any vertex in the set leaves the remaining vertices of the set in the same connected component. (AT-free graphs have asteroidal number at most 2.) In this article, we characterize graphs of bounded asteroidal number by means of a vertex elimination ordering, thereby solving a long-standing open question in algorithmic graph theory. Similar characterizations are known for chordal, interval, and cocomparability graphs.

Быстрая оценка минимального расстояния между двумя конфокальными гелиоцентрическими орбитами

The paper is on the problem of classification an asteroid as potentially hazardous (PHA), namely the estimation of the MOID parameter. Minimum Orbital Intersection Distance describes the minimal distance between two confocal heliocentric orbits. Analytical, numerical and hybrid methods used for the MOID estimation are reviewed. A brief description of the K. V. Kholshevnikov and G. F. Gronchi analytical methods, which are considered to be classical, is given. The task of calculating the MOID parameter for a large number of asteroids (more than 10,000) with a maximum calculating speed and the ability to parallelize the process is set. A numerical method based on geometrical considerations concerning the location of the bodies on their orbits is proposed. Let us consider two bodies A and E. Since only the minimum distance between two orbits is required, the information on the actual position of the bodies on their orbits is insignificant. The idea is to calculate one full revolution of the body A. For each position of body A the corresponding position of the body E is calculated under the following assumption. Consider a plane P , comprising the body A and the Sun. Therefore, plane P is perpendicular to the orbital plane of the body E. Of the two points at which the plane P intersects the orbit of the body E, E is considered to be at the point that is the nearest the body A. Thus, the position of the body E will depend on the position of the body A. As a result, from the geometric assumptions on the triangle formed by the Sun and two bodies, the distance between A and E is calculated. When one complete revolution of the body A with a certain step is calculated, we receive a set of the distances between two orbits, from which we can identify the areas of the local minima of the discrete representation of the distance function (the distance between the orbits of A and E). Then, the procedure of tuning is carried out to verify and precise the values of local minima of discrete representation of the distance function. As a result, the smallest value of the local minima is considered to be the estimation of the Minimum Orbital Intersection Distance (MOID) takes. Pros of the suggested method are as follows: high speed and adjustable calculation accuracy, the suitability to the use of parallel computing. Comparative tests of the described method were carried out. The results received are consistent with the classical G. F. Gronchi method.

How many assay probes to find one ore-bearing asteroid?

The number of ore-bearing asteroids could well be small and remote telescopic techniques are inadequate to identify such asteroids confidently. Finding an asteroid that can be profitably mined requires proximate observations from assay probes. Here we use a simple statistical approach to estimate the number of assay probes, Nassay, needed to find at least one ore-bearing asteroid at a high confidence (90%, 95%, and 99%). We present results for a wide range of values of the probability of an asteroid being rich in the resource of interest, Prich. We find that Nassay depends strongly on Prich, for likely values of Prich (<0.5). For a plausible value of Prich~0.1 then to obtain 90% confidence that at least one ore-bearing asteroid is found, Nassay=22, and for 99% confidence Nassay=44. A factor two increase in Prich roughly halves Nassay, while for Prich~0.5, Nassay (90%)=4. Hence any improvement in asteroid characterization prior to sending probes to its proximity would be an effective way to cost-effectively search for valuable resources among the asteroids. Some possibilities for doing so are briefly discussed.

A possible mechanism to explain the lack of binary asteroids among the Plutinos

Context. Binary asteroids are common in the solar system, including in the Kuiper belt. However, there seems to be a marked disparity between the binary populations in the classical part of the Kuiper belt and the part of the belt in the 3:2 resonance with Neptune – i.e., the region inhabited by the Plutinos. In particular, binary Plutinos are extremely rare. Aims. We study the impact of the 3:2 resonance on the formation of Kuiper belt binaries, according to the Nice model, in order to explain such phenomenon. Methods. Numerical simulations are performed within the 2 + 2 body approximation (Sun/Neptune + binary partners). The MEGNO chaos indicator is used to map out regular and chaotic regions of phase space. Residence times of test (binary) particles within the Hill sphere are compared inside and outside of the 3:2 resonance. The effect of increasing the heliocentric eccentricity of the centre of mass of the binary system is studied. This is done because mean-motion resonances between a planet and an asteroid usually have the effect of increasing the eccentricity of the asteroid. Results. The stable zones in the MEGNO maps are mainly disrupted in the resonant, eccentric case: the number of binary asteroids created in this case is significantly lower than outside the 3:2 resonance. Conclusions. In the 2 + 2 body approximation, the pumping of the eccentricity of the centre of mass of a potential binary destabilises the formation of binaries. This may be a factor in explaining the scarcity of binaries in the Plutino population.

Obliquity, precession rate, and nutation coefficients for a set of 100 asteroids

Context. Thanks to various space missions and the progress of ground-based observational techniques, the knowledge of asteroids has considerably increased in the recent years. Aims. Due to this increasing database that accompanies this evolution, we compute for a set of 100 asteroids their rotational parameters: the moments of inertia along the principal axes of the object, the obliquity of the axis of rotation with respect to the orbital plane, the precession rates, and the nutation coefficients. Methods. We select 100 asteroids for which the parameters for the study are well-known from observations or space missions. For each asteroid, we determine the moments of inertia, assuming an ellipsoidal shape. We calculate their obliquity from their orbit (instead of the ecliptic) and the orientation of the spin-pole. Finally, we calculate the precession rates and the largest nutation components. The number of asteroids concerned leads to some statistical studies of the output. Results. We provide a table of rotational parameters for our set of asteroids. The table includes the obliquity, their axes ratio, their dynamical ellipticity Hd, and the scaling factor K. We compute the precession rate ˙ ψ and the leading nutation coefficients Δψ and Δe. We observe similar characteristics, as observed by previous authors that is, a significantly larger number of asteroids rotates in the prograde mode (≈60%) than in the retrograde one with a bimodal distribution. In particular, there is a deficiency of objects with a polar axis close to the orbit. The precession rates have a mean absolute value of 18 �� /y, and the leading nutation coefficients have an average absolute amplitude of 5.7 �� for Δψ and 5.2 �� for Δe. At last, we identify and characterize some cases with large precession rates, as seen in 25143 Itokawa, with has a precession rate of about −475 �� /y.

A multidomain approach to asteroid families’ identification

Previous works have identified families halos by an analysis in proper elements domains, or by using Sloan Digital Sky Survey-Moving Object Catalog data, fourth release (SDSS-MOC4) multi-band photometry to infer the asteroid taxonomy, or by a combination of the two methods. The limited number of asteroids for which geometric albedo was known until recently discouraged in the past the extensive use of this additional parameter, which is however of great importance in identifying an asteroid taxonomy. The new availability of geometric albedo data from the Wide-field Infrared Survey Explorer (WISE) mission for about 100,000 asteroids significantly increased the sample of objects for which such information, with some errors, is now known. In this work we proposed a new method to identify families halos in a multi-domain space composed by proper elements, SDSS-MOC4 (a*,i-z) colors, and WISE geometric albedo for the whole main belt (and the Hungaria and Cybele orbital regions). Assuming that most families were created by the breakup of an undifferentiated parent body, they are expected to be homogeneous in colors and albedo. The new method is quite effective in determining objects belonging to a family halo, with low percentages of likely interlopers, and results that are quite consistent in term of taxonomy and geometric albedo of the halo members.